1. Field of the Invention
This invention relates generally to ring lasers, and more particularly to method and apparatus for biasing a ring laser so as to avoid the lock-in effects that must be overcome to sense rotation at low angular rates.
2. Description of the Prior Art
Ring lasers are useful, practical devices because of their extreme sensitivity to non-reciprocal effects such as rotational motion. Thus, a ring laser may serve as a rotation sensor. In this application it performs the function of a gyroscope and is thus commonly referred to as a ring laser gyroscope.
A ring laser rotation sensor is based on the following principle of operation. A ring laser supports traveling-wave modes of oscillation that propagate in opposite directions around the perimeter of the ring, the geometry of which is typically a square or a triangle. In the absence of rotation, the round-trip optical path is the same for two waves propagating in opposite directions, and the generation frequency for the two waves is the same. Rotational motion, however, has the effect of making the optical path longer in one direction and shorter in the other, whereupon the two contra-directional traveling waves are generated with a difference in frequency proportional to the angular rate. If the two output beams are made to interfere by bringing them to substantially parallel coincidence upon a square-law photodetector, the photodetector generates an electrical signal at the beat frequency.
However, such devices suffer from the problem that a signal frequency is not observable unless the angular velocity exceeds a certain critical value, typically on the order of several hundred degrees per hour. Below this critical value, the beat note frequency vanishes. Because of this phenomenon, known as "lock-in," the instrument fails to respond to rotation rates less than the critical, or "lock-in" rate. Lock-in is the result of small amounts of coupling, via backscatter, between the two contra-directional traveling waves.
Many applications require that the instrument respond to rotation rates several orders of magnitude below the lock-in rate. Fortunately, this capability may be achieved by biasing the ring laser, so that even when the instrument is at rest, the two contra-directional waves are generated with different frequencies. In this way the lock-in effects are overcome, and the ring laser is sensitive to the very low angular rates sought to be sensed or measured.
Biasing is conventionally accomplished by application of a non-reciprocal effect--that is, any effect that introduces a directional anisotropy, so that optical pathlength is different for the two directions of propagation. Typically this is either a controlled rotational motion or a magneto-optic effect such as the Faraday effect or the Kerr magneto-optic effect.
The most straightforward procedure is to establish a bias that is constant in time. Then the output difference frequency is the bias frequency if the instrument is stationary, but departs from the bias value if the instrument undergoes rotation in the plane of the ring. The principal difficulty with a constant bias is the fact that bias drift cannot be distinguished from true rotation. The ring laser responds in the same fashion to the one or to the other.
It is therefore essential to stabilize a fixed bias, which requires control of those physical parameters whose variations cause the bias to shift. In the case of a fixed rotational bias, the ring cavity is rotated at a constant rate, and the constant angular (bias) rate is the bias parameter that must be stabilized. In the case of a fixed magneto-optic bias, the applied magnetic field and a material magneto-optic constant (which is temperature sensitive) are the bias parameters that must be stabilized. Control is effected by monitoring the relevant parameters (mechanical, magnetic, thermal), that is, by measuring each relevant parameter and comparing the measured value with a reference value. If the two values are unequal, a correction can be applied to null the difference.
The success of such stabilization is limited by the fact that the ring laser is extremely sensitive to variations in the bias parameters. Thus, a small change in a bias parameter produces a significant bias shift, and good bias stability depends on the ability to measure and control the relevant bias parameters to high precision. In practice it is difficult to stabilize a fixed bias generated by a non-reciprocal effect to the degree required in most applications.
The stability problems inherent in a fixed, non-reciprocal bias are partially overcome if an alternating bias is employed. According to this method, exemplified in U.S. Pat. No. 3,373,650 (Killpatrick), the bias sense is periodically reversed, between positive and negative, twice each bias cycle. Typically, the reversal is accomplished either by a small-amplitude oscillatory motion of the entire ring cavity ("mechanical dither") or by an alternating magnetic field applied to an intra-cavity magneto-optic element. An alternating bias has the advantage that the control requirements are less severe than those of a fixed bias, but has the disadvantage that the difference frequency must pass through the lock-in zone (where the instrument is insensitive to rotation) twice each bias cycle.
Another prior art bias method employs two fixed biases in the same ring cavity by generating two traveling waves in each direction, rather than one, so that there are four waves in all. Thus four distinct optical frequencies are generated by these systems, which are known variously as multi-oscillator laser gyros, four-wave systems, and four-mode systems. The objective of such systems, exemplified in U.S. Pat. No. 4,006,989 (Andringa) and U.S. Pat. No. 3,862,803 (Yntema, et al.), is to cancel errors due to bias drift by subtracting two nominally equal bias frequencies. However, a four-wave system introduces complexities and error terms not present in a two-wave system. As will be seen below, the present invention pertains solely and strictly to two-wave systems.
The control requirements for a fixed bias are substantially reduced if bias information is obtainable directly from the ring laser output. One way to do this is to generate a second pair of waves, following the method taught in U.S. Pat. No. 3,807,866 (Zingery), wherein the bias frequency is obtained by measuring the beat frequency generated by two co-directional waves. This is again a four-wave system and subject to the same difficulties of other four-wave systems. A second way to obtain bias information directly from the ring laser output is described in U.S. Pat. No. 3,879,130 (Greenstein). According to this technique, the ring laser is operated as a two-wave system, and a fixed bias is generated by means of an intra-cavity saturable absorber. Bias variations produce a characteristic change in output intensity, which can be monitored to detect a shift in the bias with respect to a reference value. This method has the disadvantage that the bias frequency is bounded below by the axial mode spacing c/p (c=velocity of light, p=ring perimeter); for small ring perimeter, the minimum bias frequency is unacceptably large.